Tropical forests have been threatened by increasing rates of deforestation or clear-cutting during the past three or more decades (E. F. Lambin, H. J. Geist, E. Lepers, Ann. Rev. Environ. Res. 28, 205 (2003)). Although deforestation, largely for conversion of land to food crops or pastures, is the major destructive force in tropical forests worldwide, other forest disturbances such as the selective harvest of timber have increased in frequency and extent (D. C. Nepstad et al., Nature 398, 505 (1999), L. M. Curran et al., Science 303, 1000 (2004)). In selective logging, a limited number of marketable tree species are cut, and logs are transported off-site to sawmills. Unlike deforestation that is readily observed from satellites, selective logging in the Brazilian Amazon causes a spatially diffuse thinning of large trees that is hard to monitor using satellite observations. Selective logging causes widespread collateral damage to remaining trees, sub-canopy vegetation and soils, with impacts on hydrological processes, erosion, fire, carbon storage, and plant and animal species.
There is surprisingly little known about the extent or impacts of selective logging throughout the tropical forests of the world, including the Amazon Basin. A survey of sawmills in the Brazilian Amazon suggested that 9,000-15,000 km2 of forest had been logged in 1996-97 (D. C. Nepstad et al., Nature 398, 505 (1999)). The large uncertainty in this reported area resulted from necessary assumptions of the wood volume harvested per area of forest. Sawmill surveys can, at best, provide only a general idea of where and how much logging occurs because most operators buy timber at the mill gate rather than harvesting the wood themselves.
Objective, spatially-explicit reporting on selective logging requires either labor-intensive field surveys in frontier and often violently contested areas, or by remote detection and monitoring approaches. Previous studies of small areas show the need for high-resolution observations via satellite. Moreover, most of the traditional analysis techniques employed for localized selective logging studies have proven insufficient for large-scale selective logging assessments. A detailed comparison of Landsat satellite observations against field measurements of canopy damage following selective logging proved that traditional analytical methods missed about 50% of the canopy damage caused by timber harvest operations (G. P. Asner, M. Keller, R. Pereira, J. Zweede, Rem. Sens. Environ. 80, 483 (2002)).